The present invention concerns a process and a device for the conveyance and measuring of liquids. The invention also concerns the use of the said process and the said device for the conveyance and measuring of medical liquids and in particular of aseptic or contaminated liquids in a peritoneal dialysis system.
It is just the medical employment which, above all, will be referred to in the following but, for a person skilled in the art, it will become clear that the invention will be applicable to the conveyance and measuring of any whatsoever liquid.
It is known that peritoneal dialysis is a technique for the treatment of chronic renal failure which utilizes the peritoneum as a filtration membrane and the exchange between the blood irrigating the said peritoneum and liquid introduced in the abdominal cavity of the patient by means of a catheter. For each cycle, there must be known exactly the volume of the introduced dialysis solution (aseptic liquid, named dialysate) and, likewise, it must be known the volume removed at the end of each cycle (eventually contaminated liquid) in order to be in the condition to judge the quality of the executed dialysis by calculating the obtained ultrafiltration volume. As is generally known, ultrafiltration is the difference between the volumes of the drained dialysate and the introduced dialysate and corresponds to the volume of water removed from the patient. Ultrafiltration is one of the objects of the dialysis and it must be determined precisely and reproducibly.
It is evident in the art, that for the purpose of a precise and reproducible balancing of the volumina of the conveyed liquids, usually employed peristaltic pumps are only applicable in a qualified sense.
In WO-A 89/03 696, a blood purification apparatus for extracorporeal dialysis is described which comprises means for withdrawing blood from a patient into a blood conduit, a blood purification module in fluid flow communication with the said conduit, a blood reservoir for receiving blood from the said module and means for returning blood from the said reservoir to the patient via the blood purification module. The blood flow is actuated by a pump assembly which includes a piston movable in a cylinder. The cylinder is subdivided into a first and a second pump chamber which are separated one from another by a flexible wall. The piston acts in the first pump chamber a hydraulic fluid to distend or relax said flexible wall. In a second chamber, a resiliently walled blood reservoir is provided so that, when the piston acts upon the hydraulic fluid in the first chamber, the flexible separation wall distends and exerts a sufficient squeezing action on said resiliently walled blood reservoir in order to overcome the resistance of the blood circuit, to return the blood to the patient. As the piston retracts, blood flows into the resiliently walled blood reservoir as a result of the negative pressure exerted upon the hydraulic fluid in the first chamber.
Another embodiment for the pump assembly in extracorporeal dialysis is disclosed in EP-A 0 105 845. Therein, the pump assembly consists of a pumping apparatus associated to a pneumatic machine. Said pneumatic machine comprises a rigid housing dividing the cavity into two separated chambers, one connected to the dialyzer and the other connected to the pneumatic machine. The pressure or vacuum created in the other chamber by the pneumatic machine, deflects alternatively the resilient membrane in and out of this chamber causing the volume of the one chamber to vary, thus causing the one chamber to pump the blood in and out of the patient to and from the dialyzer.
However, by the interconnection of such a resilient wall or resilient membrane, a constant pressure upon the liquid on the respective gas cushion cannot be obtained since the actual force and hence the pressure exerted upon the liquid or gas cushion will depend from the actual stretch of the resilient wall or membrane, that is to say, isobaric conditions are not reached.